1. The Field of the Invention
The present invention is directed to a reactivity computer and method that allows the period for determining control rod worth to be minimized and, more particularly, to a system that verifies that control rod worth can be determined before indicating to an operator that the control rods in a nuclear reactor core can be moved.
2. Description of the Related Art
A requirement has long been in force that, at startup of each new operations cycle of a nuclear power plant, measurements be made to verify that the nuclear characteristics of an installed reactor core meet certain previously established acceptance criteria. Among the characteristics to be checked are certain differential and integral reactivity worths, such as control bank worths (the neutron absorbtion value of the control rods in a bank), moderator temperature coefficients, and the like.
It has also long been common practice to use an analog or digital "reactivity computer" to relate time dependent changes in core neutron flux level to instantaneous core reactivity. The well known algorithms used in making the relation are referred to as the "kinetics" equations. In practice, a continuous electrical signal derived from one or more neutron sensitive detectors and believed to be proportional to the neutron flux level in the core is input to an analog or digital computer programmed to solve the point kinetics equations and an output, usually analog, representative of core reactivity is generated by the computer and is supplied to a recording device, such as a strip chart recorder.
Methods of analysis and evaluation of the recorded reactivity traces developed over the years allow the user to extract from the recordings the values of various differential reactivity worths which are to be measured. These methods involve, among other things, the identification of certain nominally linear segments of the strip chart traces and the extrapolation of fits to the linear segments into portions of the record where the traces are not linear. If the fits of the lines on which the extrapolations are based are not correct, either because of analyst error or because the nominally linear segment of the strip chart trace is not truly linear, the values of the reactivity worths extracted will be incorrect, erroneous conclusions regarding the characteristics of the .re will be drawn and conceivably a potentially hazardous condition could pass undetected.
Certain nuclear properties of the large reactor cores found in a modern nuclear power plant can give rise to changes in the neutron flux distribution or shape in the core in response to a perturbation in the core, typically control rod movement. This flux redistribution temporarily "confuses" the reactivity computer and results in phenomena called "overshoots" and "undershoots" in the recorded reactivity traces. The "overshoots" and "undershoots" cause segments of the reactivity traces that should be linear to be initially non-linear and, if not properly accounted for, give rise to the errors in the extrapolation noted above and to the consequences of such errors. The suspicion always exists that, even if the analyst conscientiously follows the established methods of analysis and evaluation, the results obtained may be incorrect. Special test data illustrated in FIG. 1, obtained during the recent startup of a nuclear power plant after refueling have yielded a clear and unambiguous demonstration that indeed conventional analysis and evaluation can yield detectably incorrect results.
During a period of reactivity measurement, the boron concentration in the reactor coolant system is continuously reduced at a nominally constant ate by normal dilution operations. The reduction in boron concentration gives rise to a continuous, linear increase in core reactivity which would appear on a single neutron detector reactivity trace as a line of constant positive slope. Movement of the control bank to compensate for the effects of continuous boron concentration reduction results in the abrupt reduction in core reactivity. Since the control rods move very quickly, with a time constant of about 1 second, and since the reduction in boron concentration is virtually linear in time, it is normally expected that the reactivity trace would consist of a linearly increasing segment, a nearly instantaneous negative step and another linearly increasing segment with the same slope as the first segment. Neutron flux redistribution in the reactor core in response to control rod movement gives rise to the otherwise unexpected non-linear behavior of the indicated reactivity trace in the time period following movement of the control rods.
FIG. 1 shows the indicated differential reactivity worth of a bank of control rods as seen by each of two neutron detectors which see different regions of the reactor core and which are affected differently by the flux redistribution in the reactor core caused by control rod movement. The fact that the two plots 10 (bottom detector) and 12 (top detector) of differential reactivity worth differ is clear evidence that at least one, and probably both, indicated measurements are invalid and ought not to be used to verify that the reactor can be operated safely. Therefore, what is needed is a device that performs the intended function of the reactivity computer in such a way that the errors noted above cannot occur, provided the test operations are carried out correctly, and that the results obtained can be demonstrated to be valid and correct within the accuracy of the values of the several physical constants that are used in solving the point kinetics equations.